Earth-moon-earth
With Earth-Moon-Earth (EME) - also English moonbounce called - is called a radio link for communication between two distant points on the earth where the moon as a passive reflector is used. This method is still used today in the amateur radio service , in all other areas it has been replaced by satellite communication .
Procedure
The technology is characterized by the high free space attenuation of the signal of 243 decibels (dB) at a frequency of 50 megahertz (MHz) up to 289 dB at 10,368 MHz, with only slight fluctuations due to the variation in the distance from the moon. The comparatively high free space attenuation is due to the moon's low reflectivity ( albedo ) for high frequencies and the long distance that the signal travels. For this reason, frequencies in the VHF range are mostly used, as antenna systems with the required high antenna gain are possible here. The radio signal takes around 2.5 seconds to travel around 770,000 km to the moon and back.
So that the emitted signals can penetrate the atmosphere, the signals for EME must be in the wavelength spectrum between about 20 meters (15 MHz) and about 7 millimeters (40 GHz), the so-called radio window .
Earth-Venus-Earth communication, in which the planet Venus is used as a reflector, is a similar method, but with even higher free space attenuation .
story
The technique was developed by the US military shortly after World War II in Project Diana . The first successful reception of echoes from the moon was made by John Hibbett DeWitt on January 10, 1946 in Fort Monmouth , New Jersey . The method then found practical application, for example, with the US Navy , which used it for telex connections between Pearl Harbor (Hawaii) and the headquarters in Washington . In the days before satellite communications , a radio link without the uncertainties of ionospheric propagation was something revolutionary.
Later the technology was also used by non-military users; In 1953, radio amateurs received signals from the moon for the first time . On February 17, 2013 a Russian radio amateur (callsign RW3BP) received his echoes from the moon at the very high frequency of 47 GHz.
Antennas
In amateur radio, EME radio operations are carried out on frequencies in the VHF range (mainly on the amateur radio bands 144 MHz, 432 MHz, 1296 MHz up to 10 GHz). At 144 MHz, groups of long yagi antennas are mainly used. With a transmission power of around 750 watts, you can just perceive your own echoes from the moon with four antennas, each approx. 5 m long in the telegraph .
Parabolic mirrors are used at the higher frequencies . At 1296 MHz, a 4 m parabolic mirror has around 35 dB antenna gain compared to a dipole antenna , so that around 100 watts of transmission power is sufficient to hear its own echoes. For example, if the other station has a 6 m parabolic mirror, you can work with a 2 to 3 m mirror yourself in order to hear each other.
polarization
Linearly polarized Yagi antennas are commonly used up to a frequency of 432 MHz. Horizontal polarization is common . Since the polarization can rotate both through the optical axis in relation to the location, for example approx. 90 ° between Europe and the USA, as well as through electrical effects such as the Faraday rotation , a Yagi antenna offers the option between horizontal and vertical polarization can be switched, significant advantages. The maximum attenuation of the signals is then at 45 ° polarization at −3 dB.
Since parabolic antennas are usually used from 1296 MHz, a circularly polarized exciter can be used. This can be a patch feed or a septum feed, for example. The agreement applies worldwide that the moon transmits clockwise and clockwise circularly. Since the parabolic antenna reflects the excitation signal, the polarization at the feed in the antenna is connected exactly inversely to the radiation.
At 10 GHz, US stations still transmit horizontally and European stations transmit vertically. Because of the high mechanical precision required, circular exciters at 10 GHz are still in the minority.
Operating modes
Since 2003 a paradigm shift has emerged . Digital operating modes such as WSJT gradually supplanted classic Morse code telegraphy . As a result, EME can be carried out with even less signal strength.
software
In order to calculate the position of the moon in relation to your own position on earth, there is a freely available software from Australia by Doug McArthur, VK3UM. With the VK3UM Planner both the moon position can be displayed and a moon window between, for example, a station in Germany and a station in Australia can be calculated. With the VK3UM Calculator you can calculate how strong your own echo can be received via the moon, for example.
literature
- Edward P. Tilton: Lunar DX on 144 Mc.! In: QST, March 1953, pp. 11f., 116.
Web links
- Earth-Moon-Earth in the amateur radio wiki of the German Amateur Radio Club and the Adacom Association for Amateur Data Radio e. V.
- EME on 23 cm or 1296 MHz: The associated technology - antenna, transmitter, receiving system
- Audio examples for EME signals
- More information on 10 GHz EME
- Example of how a circular 10 GHz feed works
Individual evidence
- ↑ Paul Kelley, N1BUG: EME (moonbounce). (No longer available online.) In: N1BUG Web. Archived from the original on July 26, 2013 ; accessed on January 10, 2021 (English).
- ↑ Der Funkamateur, April 2013, p. 458.
- ↑ http://www.vk3um.com/software.html